lib/diglib: calculate the total size of topo file to get the correct off_t_size

[HuidaeCho]

Creating a new PR for #3163 (accidentally merged instead of rebase).

off_t sizes for coor and topo files can be different. For example, from an EPSG:4326 location,

> v.mkgrid map=grids_00625deg position=coor coor=-180,-90 box=0.0625,0.0625 grid=2880,5760
> d.mon wx0
> d.vect grids_00625deg
ERROR: Unable to seek: Invalid argument

> ls -al .../vector/grids_00625deg
-rw-r--r-- 1 hcho users 1708966098 Sep 17 22:35 coor
-rw-r--r-- 1 hcho users         95 Sep 17 22:33 dbln
-rw-r--r-- 1 hcho users        173 Sep 17 22:33 head
-rw-r--r-- 1 hcho users        260 Sep 17 22:33 hist
-rw-r--r-- 1 hcho users 6706995200 Sep 17 22:45 sidx
-rw-r--r-- 1 hcho users 2173694570 Sep 17 22:45 topo

coor_size is less than PORT_LONG_MAX (2^31 - 1 = 2147483647; max int), but the topo size is greater than that. Setting ptr->off_t_size = 4 in

grass/lib/vector/diglib/plus_struct.c

Lines 677 to 682 in fd59e94

	if (ptr->coor_size > (off_t)PORT_LONG_MAX) {
	/* can only happen when sizeof(off_t) == 8 */
	ptr->off_t_size = 8;
	}
	else
	ptr->off_t_size = 4;

causes an integer overflow and ptr->Isle_offset is read as -2121341854 in

grass/lib/vector/diglib/plus_struct.c

Line 641 in fd59e94

if (0 >= dig__fread_port_O(&(ptr->Isle_offset), 1, fp, ptr->off_t_size))

This PR calculates the total size of topo file to get the correct off_t_size. Before this PR, the size of coor file was used to determine this offset size for topo file.

[metzm]

Thanks for this bugfix!

[echoix]

This PR is approved and reviewed. Is there anything that should block merging this?